Many applications require pressure measurement in high temperature, hostile environments. These applications include, among others: pressure measurement in gas turbine combustors; and pressure measurement inside the cylinders of internal combustion engines. In both applications, the pressure measurement can be used to help verify desired combustion stoichiometry and proper fuel/air mixing. The pressure measurement can also identify the magnitude of undesirable pressure pulsations in gas turbines and proper spark advance and identification and correction for knock in internal combustion engines.
Present methods to measure pressure in gas turbines and internal combustion engine cylinders are inadequate and unreliable mainly because of the high temperature environment. For example, combustion temperatures generally exceed 2500°F.
Most transducers contain a diaphragm clamped (welded) around the circumference of the body. The center of the diaphragm deflects in proportion to the applied pressure. Various sensors may be used to measure diaphragm deflection, including electronic sensors.
Electronic sensors such as strain gages, capacitance gages, and piezoelectric devices all require lead wires to be bonded to the sensor. The lead wires and sensor materials are dissimilar and over time and over many thermal cycles the wire bonds eventually fail. On the other hand, fiber optic sensors such as Fabry-Perot or microbend may also be configured to measure diaphragm deflection. Fiber optic sensors use glass optical fiber to bring light to the diaphragm and the diaphragm deflection modulates the light signal. These sensors can survive higher temperatures than electronic sensors because no wire bonds are needed.
Nevertheless, the diaphragm material determines the high temperature performance limit of the pressure transducer. High temperature steel alloys such as Inconel or Hastelloy do not maintain their yield strength at combustion temperatures. The modulus of the materials decreases as temperature increases and the cyclic stress-strain characteristics become unrepeatable at high temperatures. The result is unrepeatable diaphragm deflection with pressure, which causes the pressure transducer to be unreliable.
One method to circumvent these problems is to install a tube connected and sealed at one end to the combustion chamber (or engine cylinder) and at the other end to the pressure transducer. The purpose of the tube is to permit the transducer to stand off from the hot zone. In some applications this approach is adequate, but for engines and turbines, the pressure tube may introduce a pressure drop. Additionally, it may attenuate high frequency pressure transients, which are important to measure when pressure pulsations and knock must be detected. Finally, a tube is not a viable solution for an internal combustion engine because the volume of the tube changes the cylinder volume, which in turn affects the combustion conditions and spark timing.
If a pressure transducer with deflecting diaphragm is used in an internal combustion engine, the diaphragm must be protected from transient heating from the flame front during the ignition portion of each pressure cycle. One method
for diaphragm protection is disclosed in U.S. Pat. No. 5,385,053 to Wlodarczyk et al. It discloses using a porous metal shield as a cap to cover the diaphragm. The shield conducts heat away from the flame. Because of its porosity, it transmits pressure changes directly to the diaphragm with no time delay or attenuation. However, over long time periods the small pores in the shield will be become coated with carbon and eventually plug, causing it to become ineffective.
The present invention is directed to overcoming one or more of the problems set forth above.